Method And Apparatus For Magnetic Resonance Imaging Thermometry

ABSTRACT

A system and method to analyze image data. The image data may be used to assist in determine the presence of a feature in the image. The feature may include a bubble.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.16/282,213, filed Feb. 21, 2019 and includes subject matter similar tothat disclosed in concurrently filed U.S. patent application Ser. No.16/282,193, now Issued U.S. Pat. No. 11,276,174, Issued Mar. 15, 2022and U.S. patent application Ser. No. 16/282,219. The entire disclosureof the above applications are incorporated herein by reference.

FIELD

The present teachings relate generally to an imaging analysis method andsystem, and particularly to a method and system for bubbledetermination.

BACKGROUND

The statements in this section merely provide background informationrelated to the present disclosure and may not constitute prior art.

Imaging techniques have been used to image various portions of the humananatomy. Imaging techniques include ionizing radiation, producing fieldsrelative to a human anatomy, etc. Various types of imaging includeimaging by producing fields relative to an anatomy, such as a magneticfield (e.g. magnetic resonance imager (MRI)), and sensing a change inatomic particles of the anatomy induced by the fields. Determining atemperature within an image is performed with various techniques, suchas those used in the Visualase® laser ablation system including anMRI-guided, minimally invasive laser ablation system sold by Medtronic,Inc. having a place of business in Minnesota, USA.

SUMMARY

During various procedures, a therapy may be applied to a subject. Thesubject may include a non-living structure or system, such as an airframe or other construct. Additionally, or alternatively, the subjectmay include living subjects, such as human subjects. Regardless, invarious embodiments, an instrument may be used to apply a therapy to thesubject. The therapy may include an application of a heat source orcreating heat at a selected location within the subject.

During application of heat, a selected treatment may be carried out,such as an ablation. Ablation may occur within a subject, such as todestroy or remove selected tissue, such as a tumor. In variousembodiments, an ablation instrument may be positioned within a brain ofa subject to destroy a tumor therein.

A heat application catheter may be positioned within a subject. Forexample, a cold laser fiber (CLF) system may be used to deliver thermalenergy to a tissue. Such CLF systems include those disclosed in U.S.Pat. No. 7,270,656, incorporated herein by reference. The CLF may beused to deliver thermal energy to a selected portion of a subject toablate tissue within the subject. During ablation, it is selected todetermine a temperature near the ablation instrument at a selected sightwithin the subject. In various embodiments, an image may be acquired ofthe subject including a region within or near the ablation instrument tocalculate or to determine the temperature within the subject.

When acquiring an image of the subject, various items within the imagemay cause variations within the determined temperature. For example, abubble may form in a subject during an ablation procedure. During theablation procedure, the formation of a bubble may allow or require adetermination of a temperature in an area of the bubble and/or adjacentto the bubble. The bubble, and a phase shift in selected imagemodalities (e.g. magnetic resonance imaging), may create a distortion orartifacts that may be accounted for to determine a selected temperature.Accordingly, a system and method is disclosed to detect and/or correctfor phase distortion caused by a bubble to determine a temperaturewithin an image at a selected location. The selected location mayinclude the position of the ablation instrument.

Further areas of applicability will become apparent from the descriptionprovided herein. It should be understood that the description andspecific examples are intended for purposes of illustration only and arenot intended to limit the scope of the present disclosure.

DRAWINGS

The drawings described herein are for illustration purposes only and arenot intended to limit the scope of the present disclosure in any way.

FIG. 1 is an environmental view of a suite, in various embodimentscomprising a surgical navigation system and/or imaging system and/orablation system, according to various embodiments;

FIG. 2 is a schematic illustration of a subject and an instrumentpositioned relative thereto, according to various embodiments;

FIG. 3A is an exemplary image of a subject with an instrument positionedwithin tissue thereof, according to various embodiments;

FIG. 3B is an image of a subject with an instrument therein having a lowintensity region near the instrument;

FIG. 4 is a flowchart of a method to determine a bubble and/orcompensate therefor;

FIG. 5 is a detailed flowchart for a method of generating a bubble imagelibrary;

FIG. 6 is an example of bubble images in a bubble image library;

FIG. 7 is a schematic illustration of a comparison and identification,according to various embodiments;

FIG. 8 is a schematic illustration of a comparison method, according tovarious embodiments;

FIG. 9 is a flowchart that details the method of bubble detection andcompensation of FIG. 4 , according to various embodiments;

FIG. 10 is a flowchart that details of a method to determine a region ofinterest, according to various embodiments

FIG. 11 is a flowchart illustrating a method of determining a bubble inan image, according to various embodiments; and

FIG. 12 is an exemplary application of the method illustrated in theflowchart of FIG. 11 .

DETAILED DESCRIPTION

The following description is merely exemplary in nature and is notintended to limit the present disclosure, application, or uses.

With reference to FIG. 1 , a procedure may be performed, in variousembodiments, with a navigation system 20. The procedure can be anyappropriate procedure, such as an ablation procedure, a neuralprocedure, spinal procedure, and orthopedic procedure. The navigationsystem 20 can include various components, as will be discussed furtherherein. The navigation system 20 can allow a user 25, such as a surgeonto view on a display 22 a relative position of an instrument 24 to acoordinate system. The coordinate system can be made relative to animage, such as in an image guided procedure, or can be registered to apatient only, such as in an imageless procedure.

A procedure, as discussed further herein, can be performed using orbeing assisted with image data. The image data can be image dataacquired of a patient 28 using any appropriate imaging system, such as amagnetic resonance imaging (MRI) system 26. The MRI imaging system 26can be used to acquire selected image data, and/or other types of datasuch as diffusion data relating to the patient 28. The image data of thesubject 28 may include selected types of data, including magnitude andphase data. The various types of data can be used to create images forviewing on the display 22. The image data can be used by the user orsurgeon 25, such as during a selected procedure whether or not anavigated procedure. Navigation and imaging systems may include those asdisclosed in U.S. Pat. No. 8,340,376, issued Dec. 25, 2012, incorporatedherein by reference in its entirety.

The subject 28 may be a human patient, in various embodiments. It isunderstood, however that the subject 28 need not be a human. Further,the subject need not be a living subject. It is understood, that varioussystems of constructs (e.g. air frames, test systems, mainframes, etc.).Accordingly, it is understood by one skilled in the art that the subjectdisclosure is not limited to only a human subject.

The navigation system 20 can be used to navigate or track instrumentsincluding: catheters (e.g. ablation and/or delivery), probes, needles,guidewires, instruments, implants, deep brain stimulators, electricalleads, etc. The instrument 24 can be used in any region of the body.Also, any appropriate information about the instrument 24 can bedisplayed on the display 22 for viewing by the surgeon 25.

Although the navigation system 20 can include an exemplary imagingdevice 26, one skilled in the art will understand that the discussion ofthe imaging device 26 is merely for clarity of the present discussionand any appropriate imaging system, navigation system, patient specificdata, and non-patient specific data can be used. Image data can becaptured or obtained at any appropriate time with any appropriatedevice.

The navigation system 20 can include the optional imaging device 26 thatis used to acquire pre-, intra-, or post-operative or real-time imagedata of the patient 28. The illustrated imaging device 26 can be, forexample, a magnetic resonance imaging device (MRI). Other imagingdevices can include an x-ray C-arm having an x-ray source and an x-rayreceiving section, computed tomography systems, O-arm® imaging system,etc. The imaging device 26 can be provided to acquire image data of thepatient 28 prior to or during a procedure for diagnosis of the patient28.

Although FIG. 1 illustrates an environmental view showing both thepatient, surgeon, navigation system, and other elements, it will beunderstood that this is merely exemplary of all the portions that can beprovided together. For example, an electromagnetic navigation ortracking system may not be provided in a room with the imaging MRIsystem 26, but is shown in FIG. 1 for illustration and can be separatedfor use in an actual procedure.

An imaging device controller 34 can control the imaging device 26 tocapture and store the image data for use, such as in real time or forlater use. The controller 34 may also be separate from the imagingdevice 26. Also, the controller 34 can be used intra- or pre-operativelyto control and obtain image data of the patient 28.

The image data can then be forwarded from the controller 34 to aprocessor system 40 via a communication system 41. The communicationsystem 41 can be wireless, wired, a data transfer device (e.g. a CD-Romor DVD-Rom), or any appropriate system. A station 42 may be a workstation and may include the processor system 40, the display 22, a userinterface 44, and a memory 46. It will also be understood that the imagedata is not necessarily first retained in the controller 34, but may bedirectly transmitted to the workstation 42 or to a tracking system 50,as discussed herein.

The work station 42 provides facilities for displaying the image data asan image on the display 22, saving, digitally manipulating, or printinga hard copy image of the received image data. The user interface 44,which may be a keyboard, mouse, touch pen, touch screen or othersuitable device, allows a physician or user to provide inputs to controlthe imaging device 26, via the controller 34, or adjust the displaysettings of the display 22.

The processor system 40 can process various types of data, such as imagedata, provided in the memory 46 or from the imaging system 26. Theprocessor system 40 can also process navigation information, such asinformation provided from the tracking system 50. In addition,navigation processing can include determining a position (e.g. threedegree of freedom rotation and three degree of freedom spatial position)of the tracked instruments relative to the patient 28 for displayrelative to the image data 23 on the display 22. The processor system40, as discussed herein, may perform or execute instructions to performvarious types of analysis such as temperature determination, positiondetermination, etc. It will be understood, each of the processingportions can be processed by separate or individual processors or can beprocessed substantially sequentially with an appropriate processor.

The optional imaging device 26 can be any appropriate 2D, 3D or timechanging imaging modality. For example, an isocentric fluoroscopy,bi-plane fluoroscopy, O-arm® imaging devices (i.e. devices sold byMedtronic, Inc. having a place of business in Minnesota, USA),ultrasound, computed tomography (CT), T1 weighted magnetic resonanceimaging (MRI), T2 weighted MRI, positron emission tomography (PET),optical coherence tomography (OCT), single photo emission computedtomography (SPECT), or planar gamma scintigraphy (PGS) may also be used.

The image data obtained of the patient 28 can be used for variouspurposes. As discussed herein, image data can be obtained for performinga navigated procedure on an anatomy, planning an operation or procedureon an anatomy, and other appropriate reasons. For example, during aneurological procedure, it can be selected to obtain image data of abrain of the patient 28 for viewing during the procedure and, in variousembodiments, determining a temperature near a selected portion of theinstrument and/or navigating the instrument 24 relative to the imagedata 23. Further, the acquired image data can be used to plan themovement of the instrument 24 or for positioning of an implant during anoperative procedure.

The imaging device 26 can also be used to obtain various types of dataother than only image data. The various types of data can be used andoverlaid one on another to obtain an appropriate image of the anatomy.For example, a magnetic resonance image can be obtained of a portion ofthe patient 28, such as a brain 29, for viewing in a selected manner.For example, a 3-D model can be formed of the brain based upon multipleslices of MRI data for displaying on the display 22 during a tracking ofa navigated procedure.

Briefly, the navigation system 20 operates to determine the position ofthe instrument 24 relative to the subject 28 and for viewing relative tothe image 23 of the subject 28, as discussed herein. The navigationsystem 20 creates a translation map between all points in the image dataor image space and the corresponding points in the patient's anatomy inpatient space (either manually or automatically), an exemplary 2D to 3Dregistration procedure is set forth in U.S. Pat. No. 7,570,791, entitled“Method and Apparatus for Performing 2D to 3D Registration”, issued Aug.4, 2009, hereby incorporated by reference in its entirety. The pointsselected can be fiducial marks 69 that include anatomical landmarks orartificial landmarks, such as those disclosed in U.S. Pat. No.6,381,485, entitled “Registration of Human Anatomy Integrated forElectromagnetic Localization,” issued Apr. 30, 2002, hereby incorporatedby reference in its entirety. After this map is established, the imagespace and patient space are registered, that may appear and bedetermined or selected in both the image space and the subject space. Inother words, registration is the process of determining how to correlatea position in image space with a corresponding point in real or patientspace. This can also be used to illustrate a position of the instrument24 relative to the proposed trajectory and/or the determined anatomicaltarget. Registration may occur by the processes and/or system asdisclosed in U.S. Pat. No. RE42,226, issued on Mar. 15, 2011, entitledPERCUTANEOUS REGISTRATION APPARATUS AND METHOD FOR USE INCOMPUTER-ASSISTED SURGICAL NAVIGATION, incorporated in its entiretyherein by reference. In various embodiments, registration may include a2D to 3D registration such as an exemplary 2D to 3D registrationprocedure is set forth in U.S. Ser. No. 10/644,680, filed on Aug. 20,2003, now U.S. Pat. No. 7,570,791, issued Aug. 4, 2009, entitled “Methodand Apparatus for Performing 2D to 3D Registration”, hereby incorporatedby reference in its entirety.

With continuing reference to FIG. 1 , the navigation system 20 canfurther include the tracking system 50 that includes one or morelocalizers, such as an electromagnetic (EM) localizer 52, (e.g. whichcan also be referred to as a transmitter array, a tracking array,tracking coils, or coil array and can include a transmitter and/orreceiver coil array). It is understood that other appropriate localizersmay also be provide or used, such as an optical localizer. Differentlocalizers may operate in different modalities, such as optical ormagnetic field, radar, etc. The tracking system 50 is understood to notbe limited to any specific tracking system modality, e.g. EM, optical,acoustic, etc. Any appropriate tracking system modality can be usedaccording to the present disclosure. Moreover, any tracked instrument,such as the instrument 24 and/or a dynamic reference frame (DRF) 58 caninclude one or more tracking devices that operate with one or moretracking modalities. Thus, the tracking system 50 can be selected to beany appropriate tracking system, including the StealthStation® S7®surgical navigation system that offers both optical and AxiEM™electromagnetic tracking options.

One skilled in the art will understand that the coil array 52 cantransmit or receive, thus reference to the coil array 52 as atransmitter or a transmit coil array is merely exemplary and notlimiting herein. The tracking system 50 can further include a coil arraycontroller (CAC) 54 that can have at least one navigation interface ornavigation device interface (NDI) 56 for connection of the localizer 52,an instrument tracking device 67 on or associated with the instrument24, and a dynamic reference frame 58. The coil array controller 54 andthe at least one navigation interface 56 can be provided in a singlesubstantially small CAC/NDI container, if selected. The instrumenttracking device 67 may be placed or associated with the instrument 24 inany appropriate manner or position to allow for determination of aselected portion (e.g. terminal end) of the instrument 24. In variousembodiments, the tracking device 67 may include a coil positioned at ornear a terminal end of the instrument 24.

In an optional optical system, generally an optical localizer includesone or more cameras that “view” the subject space. The cameras may beused to determine position of the tracking element relative to thecameras. Tracking devices include members that are viewable by thecameras. The optical tracking devices may include one or more passive oractive portions. An active tracking device can emit a viewablewavelength, including infrared wavelengths. Passive tracking devices canreflect selected wavelengths, including infrared wavelengths.

The tracking system can be included in the navigation system 20 and mayinclude, in various embodiments, an EM localizer, which may be the coilarray 52. The EM localizer 52 can include that described in U.S. Pat.No. 7,751,865, issued Jul. 6, 2010, and entitled “METHOD AND APPARATUSFOR SURGICAL NAVIGATION”; U.S. Pat. No. 5,913,820, entitled “PositionLocation System,” issued Jun. 22, 1999; and U.S. Pat. No. 5,592,939,entitled “Method and System for Navigating a Catheter Probe,” issuedJan. 14, 1997, each of which are hereby incorporated in their entiretyby reference. The localizer may also be supplemented and/or replacedwith an additional or alterative localizer. As is understood thelocalizer 52, according to any of the various embodiments, can transmitsignals that are received by the dynamic reference frame 58, and atracking device that is associated with (e.g. connected to) theinstrument 24. The dynamic reference frame 58 and the tracking devicecan then transmit signals based upon the received/sensed signals of thegenerated fields from one or more of the localizers 52. Trackingsystems, including the optical tracking system, can include theStealthStation® S7® Surgical Navigation System, sold by MedtronicNavigation, Inc. The optical localizer can view the subject space andthe tracking devices associated with the DRF 58 and/or the instrument24.

The work station 42, either alone or in combination with otherappropriate processor systems, including the coil array controller 54and the controller 34, may identify the corresponding point on thepre-acquired image or atlas model relative to the tracked instrument 24and display the position on display 22 and relative to the image data23. This identification is known as navigation or localization. An iconrepresenting the localized point or instruments is shown on the display22 within several two-dimensional image planes, as well as on threedimensional (3D) images and models. In order to maintain registrationaccuracy, the navigation system 20 can continuously track the positionof the patient 28 with the dynamic reference frame 58. The position ofthe instrument 24 may be transmitted from the instrument tracking device67 through a communication system, such as a wired or wirelesscommunication. The tracking devices, or any other appropriate portion,may employ a wireless communications channel, such as that disclosed inU.S. Pat. No. 6,474,341, entitled “Surgical Communication Power System,”issued Nov. 5, 2002, hereby incorporated by reference in its entirety,as opposed to being coupled with a physical transmission line.

The instrument 24 used in a procedure can be any appropriate instrument(e.g., a catheter, a probe, a guide, etc.) and can be used for variousprocedures and methods, such as delivering a material, ablation energy(e.g. heat), or providing electrical stimulation to a selected portionof the patient 28, such as within the brain 29. The material can be anyappropriate material such as a bioactive material, a pharmacologicalmaterial, a contrast agent, or any appropriate material. As discussedfurther herein, the instrument 24 can be precisely positioned via thenavigation system 20 and otherwise used to achieve a protocol forpositioning and/or applying a treatment relative to the patient 28 inany appropriate manner, such as within the brain 29. The instrument 24may also include a brain probe to perform deep brain stimulation and/orablation.

With reference to FIG. 2 , the instrument 24 may be positioned withinthe brain 29 of the subject 28 such as according to various techniques,such as those disclosed in U.S. Pat. No. 7,270,656, incorporated hereinby reference. Further, the instrument 24 may include various featuressuch as an energy delivery or transfer system or mechanism 100 which mayinclude a fiber optic cable to transmit laser energy to a distal end 104of the instrument. The distal end 104 of the fiber optic member 100 maybe near a terminal end 110 of the instrument 24. The instrument 24,therefore, may generate heat or thermal energy near a tumor 114 withinthe subject 28, such as within a brain 29. Temperature near the terminalend 110, such as within the tumor 114, may be modulated by providing orvarying the amount of energy through the energy transfer system 100and/or transferring or passing a cooling medium through the instrument24. Passing a cooling medium may include providing a cooling medium to acooling medium inlet 120 that may pass through a cooling medium return124. The cooling medium can be any appropriate material, such as water,saline, or the like. Nevertheless, thermal energy may be delivered tothe subject 28 to perform a therapy on the tumor 114 within the subject28. During therapy to the subject 28, the imaging system 26 may be usedto image the subject 28 to determine a temperature at or near the end104 and/or the terminal end 110.

As discussed above, the instrument 24 may be tracked relative to thesubject 28, such that the position of the distal end 110 and/or the endof the energy delivery system 100, may be determined. Accordingly,images acquired with the imaging system 26 may be registered to thesubject 28 and/or to the instrument 24. This allows the navigatedposition of the instrument 24 to be determined relative to the imagesacquired of the subject 28. The position of the instrument 24 may bedisplayed on the display device 22, such as with a graphicalrepresentation 24 i′ displayed on the display system 22, such assuperimposed on the image 23.

During an ablation procedure, as illustrated in FIG. 1 , the user 25 mayapply energy to the subject 28 with the instrument 24 at a selected rateor time to heat a portion of the subject. During the heating, a heatingimage is acquired at a selected rate. For example, heating images may beacquired at a rate of about every five seconds, every ten seconds, orany selected period of time. Accordingly, during the application ofthermal energy to the subject 28, heat images are acquired to determinethe temperature at the location of the instrument within the subject 28.

A heat image may be an image acquired with the imaging system 26 fordetermining a temperature within the subject 28. The heat image mayinclude various information, such as diffusion information or relaxationtimes, or phase changes that may be analyzed to determine a temperatureand/or a temperature change from a prior heat image. Thus, the heatimage may be used to determine a temperature or temperature change on apixel or voxel basis relative to a prior image or alone. Thus, a heatimage may include an image acquired of the subject 28 for determining atemperature therein.

A heat image may be displayed on the display 22, or any otherappropriate display. For example, a heat image may be displayed on thedisplay device 22, as illustrated in FIG. 3 . The heat image may includea first heat image 150. The first heat image may include an image of thebrain 29 as the image 23. The heat image 150 may also include image dataor an image of the instrument 24 as the instrument 24 a. It isunderstood that the instrument 24 may appear according to differentshapes or geometries based upon the particulars of the instrument 24,and the illustration as one or a plurality of legs in FIG. 3A is merelyexemplary. However, the heat image 150 may be displayed for viewing bythe user 25 to illustrate substantially the magnitude in the image. Theheat image 150 may be slice, such as an MRI image slice, where eachvoxel or pixel includes an intensity, where a higher intensity is alighter color and a lower intensity is a darker color. The first heatimage 150 may be a baseline or first heating image. In variousembodiments, therefore, a second heating image may be acquired.

With reference to FIG. 3B, a second heat image 160 is illustrated. Theheat image 160 may also illustrate the instrument 24 a. Near or adjacentto the instrument 24 a is a dark region or low intensity region 166. Thelow intensity region 166 may be a bubble that is formed near or adjacentto the instrument 24 within the subject 28. The low intensity region 166may appear in the heat image 160 as a dark or low intensity portion nearthe instrument 24 a. The identification of the low intensity region 166as a bubble, however, may be difficult with only viewing the displaydevice 22. Moreover, a temperature at the portion including the bubbleor low intensity region 166 may be calculated even with the presence ofthe low intensity region 166, as discussed further herein.

The bubble, without being limited by the theory, may be caused by heatcaused in various tissues or materials. The materials may cause gas toform within a volume. The volume may be bounded by the material in whichthe instrument 24 is placed. The bubble, therefore, in the anatomy maybe caused by various local conditions therein. In an image, such as aMRI image, as discussed herein, the bubble may be a region devoid ofsignificant signal due to low proton density and/or rapid motion,surrounded by an image phase/frequency disturbance due to the differencein magnetic susceptibility between adjacent tissue and the bubblevolume. The bubble in this context may appear due to the conditionsassociated with a selected therapy to the subject, such as heat. Aspecific size and constitution of a given bubble depends on a localenvironment (e.g. tissue) as well as the therapy (e.g. heating)conditions.

As discussed further herein, the first heat image 150 may be acquired atany time during the application of the thermal energy to the subject 28.Further, the second heat image 160 may be any subsequent, such as animmediately subsequent image, and may also be referred to as a currentheat image. Accordingly, during the application of the thermal energy tothe subject, heat images may be acquired in sequence. Each heat imagethat is acquired that does not include a bubble may be a first orbaseline image and the subsequent image, such as an immediatelysubsequent image, that includes a bubble may be the second heat image160. It is understood, however, that the baseline or first heat image150 may also be an initial image acquired of the subject 28. In variousembodiments, the first heat image 150 may always be the first orbaseline image and every subsequent image is compared thereto fordetermination and/or to assist in determination of a bubble present withthe image.

As discussed above, with reference to FIG. 3A and FIG. 3B, a dark regionor artifact 166 may appear in the heat image 160. The region spot 166may be a bubble or other artifact feature that may reduce a confidencein a temperature determined using the heat image 160. Accordingly, withreference to FIG. 4 , a bubble determination and/or compensation method180 is illustrated. The bubble detection and/or compensation method 180may include a plurality of steps or procedures, as discussed herein,that may be included in various sub steps or procedures, as discussedfurther herein, but starts in start block 182. Accordingly, the method180 may be understood to be an overall or inclusive method or algorithmfor detecting and/or compensating for a bubble in a heat image that mayinclude various subroutines or elements that includes more than onestep, as discussed herein. Further, it is understood that the method 180may be implanted as instructions that are executed by a selectedprocessor system, such as the processor system 40. The method 180 may besubstantially automatically executed when a selected heat image orcomparison image is accessed or acquired.

Initially, a bubble image library may be generated in block 188.Generation of the bubble image library may not be required for thedetection and compensation method 180, but may be included for clarityand completeness for the current discussion. Thus, a library may begenerated such as in real time and/or prior to performing of a selectedprocedure, such as an ablation procedure, as discussed above.

Regardless of whether the bubble library is generated immediately beforeor at a prior time, the bubble image library may be accessed in block194. The bubble image library 188, therefore, may be stored on aselected or in a selected memory system to be accessed by a processor,such as the processor system 40 discussed above. It is understood thatthe processor system 40 may include a plurality of processors, and thedetection and compensation method 180 may be executed by a processorthat is included with, separate from, and/or in communication with theprocessor system 40. Regardless, an appropriate processor may executeinstructions to access the bubble image library in block 194. The bubbleimage library accessed in block 194 may include appropriate bubbleimages that may be based upon selected models that are used to generatethe bubble library in block 188. The bubble image library accessed inblock 194 may include more than one type of image, such as magnitudeand/or phase data. The bubble image library access in block 194 mayinclude or be generated based upon magnetic resonance imaging systems.

The bubble image library may be accessed in block 194 at any appropriatetime. It is illustrated in the method 180 as being initially accessed,however, it need not be accessed until compared to a selected image,such as during a comparison or prior to a comparison of a bubble imagefrom the bubble image library to a selected image, as discussed furtherherein.

Regardless of the timing of accessing the bubble library in block 194,accessing a current heat image in block 198 may occur. The current heatimage accessed in block 198 may be a heat image that is acquired by orat the direction of the user 25 during a selected procedure. The currentheat image is acquired to attempt to determine a temperature within thesubject 28 at or near an ablation region of the subject relative to theinstrument 24. As discussed above, the current heat image may be used todetermine a current temperature or a temperature at the time ofacquiring the heat image. Generally, the current heat image may beacquired at a selected rate, such as five seconds after an immediatelyprevious heat image. It is understood, however, that the current heatimage may be acquired at any appropriate time relative to a previousheat image, as may be selected by the user 25.

Accessing a previous heat image in block 202 may also occur. Theprevious heat image may be any appropriate previous heat image, such asan immediate prior heat image and/or any heat image acquired prior tothe current heat image. For example, during various procedures, aninitial or prior to ablation heat image may be acquired of the subject28. The previous heat image may be a heat image acquired at the initialor prior to ablation or therapy. In various embodiments, however, theprevious heat image may be a heat image that is acquired immediatelyprior to the access current heat image in block 198.

Regardless of the timing of the collection of the current heat image andthe previous heat image, the two accessed heat images may be compared inblock 210. The comparison of the current heat image and the previousheat image in block 210 may be used to generate a comparison image. Thecomparison image may be generated in any appropriate manner, asdiscussed further herein. Generation of the comparison image may attemptto determine differences between the current heat image and the previousheat image. The differences may include magnitude and/or phasedifferences between the current heat image and the previous heat image.The generated comparison image may include these differences for furtheranalysis, as also discussed herein.

The generated comparison image may then be analyzed to determine if abubble is present or possibly present in the comparison image. Invarious embodiments, the comparison image may be compared to at leastone bubble image accessed from the bubble image library to the generatedcomparison image in block 220. The comparison of the at least one bubbleimage to the generated comparison image may be done in any appropriatemanner, as also discussed herein. For example, the accessed bubble imagelibrary may include bubble images that include magnitude informationand/or phase change or drift that may be caused due to the presence of abubble. In comparing the bubble image from the access bubble imagelibrary to the generated comparison image in block 220, a determinationof whether a bubble is present in the comparison image may be made inblock 230. The determination of whether a bubble is present in thegenerated comparison image may be based upon the comparison of thebubble image from the bubble image library, as discussed further herein.In various embodiments, the comparison image may also be analyzed orcompared in a heuristic manner, such as analysis of the image with aselected system, as discussed herein.

The determination of whether a bubble is present may be made in block230 based upon the comparison in block 220. If no bubble is present, aNO-path 234 may be followed to access a current heat image in block 198.Again, accessing a current heat image in block 198 may be made at anyappropriate time, and may be a current heat image that may be after aheat image that is accessed in a first iteration. Accordingly, it isunderstood, that the method 180 may be an iterative process that may beperformed during a selected procedure, such as during an ablationprocedure on the subject 28. The current heat image that is accessed inblock 198 may be any appropriate current heat image that may be at atime between the initiation of therapy and the termination of a therapyand any appropriate intermediate point therein.

If a determination is made in block 230 that a bubble is present, aYES-path 238 may be followed. The YES-path 238 may be followed toidentify a location of the bubble comparison image in block 244.Identifying a location of the bubble in the comparison image in block244 may include identifying the bubble in the comparison image forfurther analysis and determination of the current heat image or thegenerated comparison image. Identification of the location of the bubblein block 244 may include identifying that a bubble exists and/or thepixels or voxels in the generated comparison image and/or access currentheat image that belonged to the bubble and/or are affected by thebubble. Thus, identifying location of the bubble in the comparison imagemay allow for further compensation of the presence of the bubble in thecurrent heat image, if selected.

Accordingly, after identifying the location of the bubble in block 244,a compensation determination block 248 allows for a determination ofwhether compensation will occur. The user 25 may select to compensatetemperature determination, as discussed herein, for the identifiedlocation of the bubble and/or may determine to terminate therapy for aselected period of time to allow the bubble to dissipate.

Accordingly, the compensation determination in block 248 may allow theuser to determine to not compensate and follow a NO-path 252 to performvarious selected procedures. Additionally, when the NO path 252 isfollowed the method 180 may iterate, as noted herein. Further, thebubble may only be identified in the image and identified to the user25. The identity to the user may be displayed with the image 23 and/orseparately therefrom. Thus, the method 180 may be to only identify abubble or possible bubble, in various embodiments.

When the NO path is followed 252, various other procedures or steps mayoccur. For example, pausing a procedure in optional pause block 256.After pausing the procedure in pause block 256, for a selected period oftime (e.g. about one second to about one minute, or any appropriatetime), the user 25 and/or the ablation system may again access a currentheat image in block 198. Again, the current heat image accessed in block198 may be acquired after the previous current image in block 198, suchas after the pause 256. Again, a determination of whether a bubble ispresent in one or a current heat image may be made and whethercompensation will be made in block 248. Accordingly, if compensation isnot made, the identification of the bubble and the current heat imagemay allow the user 25 to pause or allow for the bubble to dissipate. Thesystem, however, executes the method 180, may be used to automaticallyidentify whether a bubble exists within the current heat image basedupon the algorithm method 180.

The compensation determination in block 248 also allows for compensationto occur and thus a YES-path 260 may be followed. If compensation isselected in block 248, the YES-path 260 may be followed to removedistortion/artifact caused by the bubble in the current heat imageand/or other selected image, such as the generated compensation image inblock 270. Removal of the distortion or artifact caused by the bubble inthe current heat image in block 270 may be made according to selectedtechniques, including those discussed further herein, such as removingthe phase distortion and/or magnitude distortion caused by theidentified bubble at the identified location. The compensate image mayinclude the distortion or artifact removed that is generated in block270, such as through subtraction of the identified bubble.

Once the distortion is removed in block 270, a determined temperature inthe compensate image may be made in block 274. The determinedtemperature in block 274 may be used for performing the selectedprocedure, such as determining a temperature at or near the end of theinstrument 24. As discussed above, the ablation procedure may occur orproceed when a selected temperature is achieved or in attempt to achievea selected temperature. Accordingly, determining a temperature, asdiscussed herein, in the compensate image in block 274 may be used forperforming the procedure, such as an ablation procedure, on the subject28.

The determined temperature in the compensated image may then determinewhether a procedure may continue in block 278, according to selectedcriteria (e.g. temperature, duration, etc.). Determination of whetherthe procedure continues in block 278, however, may again be selectedbased upon the user 25 and/or performing of a selected procedure,including the ablation procedure.

If a determination is that the procedure is to continue, a YES-path 282may be followed. The YES-path 282 may again follow to accessing acurrent heat image in block 198. The current heat image may be againacquired at any appropriate time, such as after the identificationand/or compensation of a bubble in a previous current heat image.Accordingly, the current heat image accessed in block 198, whenfollowing the YES-path 282, may again be understood to create aniterative process of the method 180.

If selected, however, a NO-path 288 may be followed, such as theprocedure should terminate. When terminating the procedure, the NO-path288 may follow to an end block 290. Ending the method 180 may includecompleting a procedure on the subject 28, such as removing theinstrument 24, or other appropriate steps. Further ending the procedure180 at block 290 may include terminating application of energy for aselected procedure, at a selected time, restarting a procedure, or otherappropriate procedure steps.

As noted above, the method 180 may include various sub-steps orsub-routines, steps may be executed by a processor system, includingthose discussed above and herein. In various embodiments, therefore, thebubble image library may be generated in block 188. With continuingreference to FIG. 4 and additional reference to FIG. 5 , the generationof the generated bubble image library 188 is described in greaterdetail. The generated bubble image library method 188 may be performedautomatically with the processor system, such as the processor system 40and/or with input by the user 25 and/or appropriate user. Generally, thebubble image library is generated based upon forming a plurality ofbubble images based upon a model, including altering a model based uponsize and/or orientation of the bubble in an image.

The bubble library method may initiate in start block 300. Thereafter, abubble model may be generated and/or accessed in block 304. The accessedbubble model may be based upon selected information, such as a selecteddefinition of a bubble. In various embodiments a definition of a bubblemay include or be defined by Equation 1:

${\Delta f_{bubble}} = {{- r^{3}}\frac{\gamma}{2\pi}B_{0}d_{X}\frac{{2z^{2}} - x^{2} - y^{2}}{3\left( {x^{2} + y^{2} + z^{2}} \right)^{5/2}}}$

Equation 1 may be used to define the frequency shift of a bubble inhertz when the bubble exists in a substantially homogeneous structure,such as the brain 29. Equation 1 assumes or acknowledges that a bubblemay be substantially gas or air and that a difference between magneticsusceptibility between air or gas tissue may be about 9 ppm.Accordingly, the magnetic susceptibility of the air in the bubble may beabout 9 ppm less than the surrounding tissue, therefore the d_(x)=−9ppm. In various assumptions a gyromagnetic ratio is γ=42.58 megahertzper tesla. B₀ is the field strength in tesla of the imaging system 26,such as a MRI scanner. Further, r is the radius of the bubble and x, yand z are in centimeters and indicate a position of the bubble, where zis the B₀ direction. Frequency, f, is in hertz. Generally, the bubble isassumed to be substantially spherical therefore in a grid of x, y and zcoordinates the values within a bubble are defined or identified as zeroand masked out.

Accordingly, Equation 1 may be used to identify or calculate an imagemodel over a three-dimensional grid (x,y,z) locations within a slice. Asnoted above, an MRI may be used to generate the image data and the MRIimage may have a selected slice width. Accordingly, the MRI slice imagemay have a three-dimensional volume through which Equation 1 may be usedto calculate the residence frequency offset Δ f. A total frequencyoffset at a selected location (x,y,z) during an excitation pulse isgiven by Equation 2:

${\Delta{f\left( {x,y,z} \right)}} = {{\frac{\gamma}{2\pi}G_{z}Z} + {\Delta{f_{bubble}\left( {x,y,z} \right)}}}$

In Equation 2 γ is the same as noted above, G_(z) is the frequency shiftwith a slice gradient amplitude, z is the spatial location of the slice,and Δf_(bubble) is from Equation 1. Thus, given this frequency map and afrequency profile of an RF pulse in an MRI, interpolation may be used tocalculate a slice profile for each spatial location of the bubble, whichmay be denoted as (x,y,z). To determine a slice profile near a bubble,various assumptions may be made, such as a three millisecond per timebandwidth product of an RF pulse and a small excitation or flip angle(e.g. about 10 degrees to about 40 degrees, including about 25 degrees)may be assumed along with a three millimeter slice thickness.

Accordingly, a bubble image, which may also be referred to as a sliceprofile of the bubble, may be illustrated by Equation 3:

s _(TE)(x,y,z)=x(x,y,z)e ^(i2πTEΔf(x,y,z))

In Equation 3, the slice profile may be formed or advanced to anecho-time represented by TE, therefore the spatial profile given byEquation 3 may be at the echo-time of the imager. In Equation 3, theterm s(x,y,z) is the signal at the end of the excitation pulse and theexponential accounts for time passing to the echo-time. Accordingly, TEis the time past or accounts for the time past to the echo-time of thesignal such that the spatial profile is advanced to the echo-time. Thensumming across a slice profile is given by Δf(x,y,z) and allows forgenerating the slice profile of the bubble. A convolution to averagemultiple x and y location or direction spins may be made to account fora signal loss due at each of the x, y locations.

Further, it is understood that the model of the bubble may be based uponaccounting for the profile effects within the slice and/or without.Nevertheless, the bubble image may be based upon the accessed model, asdiscussed above.

The accessed model in block 304, as described above, may then be used togenerate a plurality of bubble images in block 310. The plurality ofbubble images may be based upon altering various characteristics of thebubble model. For example, a change in radius of the bubble may be usedto identify or determine various sizes of the bubbles. For example, theradius may be given in a selected dimension, such as voxels, and mayrange between about 1 voxel and about 50 voxels, including about 2voxels and about 12 voxels, and further including a discrete number ofvoxels between 2 and 12. For example, the bubble library may include 10bubbles each differing by 1 voxel with the smallest bubble having aradius of 2 voxels and the largest bubble having a radius of 12 voxels.Further, the bubble models may be rotated or angled relative to the axisof the imager, B₀ axis. Each of the bubbles of different radius may berotated a selected angle θ. The amount of rotation may be anyappropriate amount. For the bubble library, for example, each bubble mayhave in plane rotations of about −45 degrees to about +45 degrees in 15degree steps. The amount of rotation at x and z coordinates may be givenby X_(rot) and Z_(rot) in Equation 4 and Equation 5, respectively:

X _(rot) =X cos(θ)−Z sin(θ)

Z _(rot) =X sin(θ)−Z cos(θ)

Thus, each of the bubble images may include a bubble of a selectedradius and/or selected angle rotation relative to the B₀ axis. Each ofthe plurality of bubble images, therefore, may be saved in the bubbleimage library that may be accessed in block 194, as discussed above.Thus the plurality of images may be saved in the bubble library in block314 that may be accessed in block 194, as illustrated in FIG. 4 .

After saving a plurality of generated bubbles in a library in block 314,a determination of whether more bubbles are selected is made in block318. If more bubbles are selected, a YES-path 320 may be followed toblock 310 to generate a plurality of bubble images, which may be inaddition to a previous plurality of bubble images. If a determination inblock 318 is that no more bubbles are selected, a NO-path 324 may befollowed to end in block 330. The bubble image library may be formed atany appropriate time, such as prior to the beginning of a procedure,during a procedure, or at any selected time. Regardless, the bubbleimage library may be generated as discussed above and may be used duringa temperature sensing process.

With continuing reference to FIG. 5 and additional reference to FIG. 6 ,the bubble library may be formed to include bubble images that includeboth magnitude and phase differentiation. As understood by one skilledin the art, the phase in an MRI may relate to an encoding due to aresonance in light of the MRI imaging process. Generally, MRI imagingmay include both a frequency encoding and a phase encoding, to determineinformation regarding each pixel or voxel in a slice image. Accordingly,phase encoding may be used to assist in determining a temperature at aselected voxel within the image. As illustrated in FIG. 6 , the modelaccessed in block 304, may be used to generate library images. In FIG. 6, library images of a bubble of a selected radius are illustrated as amagnitude image in a first row 340 and a phase in a second row 350. Thebubble image in the bubble image library may identify gradations oramounts of change as well. As illustrated in FIG. 6 , an amount orvariation in the magnitude and phase variance may be included in thebubble image in the bubble image library and for correlation to thecomparison image, as discussed herein. The bubble library may furtherinclude the bubble model that is rotated relative to the axis B₀ 354 ofthe imaging system. Accordingly, the library images may include aplurality of images that are rotated in both magnitude and phase.

As illustrated in FIG. 6 , a first column 360 illustrates a magnitudeimage 340 a and a phase image 350 a that are parallel with the axis B₀of the imager. In a second column 364 a magnitude image 340 b and aphase image 350 b is illustrated for the bubble. Finally, in a thirdcolumn 368 the bubble is illustrated at substantially 90 degrees orperpendicular to the axis B₀ as a magnitude image 340 c and a phaseimage 350 c.

The bubble image library may include a plurality of images more than thesix illustrated in FIG. 6 , as discussed further herein. Regardless, thebubble library may include a plurality of images that allow foridentification and analysis of a heat image, as discussed furtherherein. It is understood that an identification system may furtherinterpolate between different bubble images to assist in identifying abubble in a current heat image or comparison image.

With continuing reference to FIG. 4 and additional reference to FIG. 7 ,the heat images that may be accessed in blocks 198 and 202, may besimilar to the heat images illustrated in FIGS. 3A and 3B. Accordingly,a previous heat image 150 and a current heat image 160 are illustrated.The current heat image 160 may be recalled in block 198 while theprevious heat image 150 may be recalled or accessed in block 202, asillustrated in FIG. 4 .

The two images may be compared to one another in block 210, as discussedabove. To compare the two images to one another a ratio may be madebetween the current heat image 160 and the previous heat image 150. Thatis, the current heat image 160 may be divided by the previous heat image150. In dividing the current heat image 160 from the previous heat image150, a ratio of each of the voxels or pixels within the current heatimage 160 may be determined. During acquisition of image data of thesubject 28, the subject 28 may be held substantially fixed relative tothe imaging system 26. Accordingly, images may be acquired over time ofthe subject 28 that may be substantially registered to one another andin series. Accordingly, a pixel or voxel location in the current heatimage 160 may be known relative to a pixel or voxel in the prior heatimage at the same position. Thus, a ratio between the two may bedetermined. It is understood that other appropriate differences orcomparisons may be made, and a ratio is merely exemplary. Nevertheless,the ratio of the current heat image 160 to the previous heat image 150may result in resultant images in column 380, illustrated in FIG. 7 .

The resultant images or generated comparison images may include amagnitude generated comparison image 384 and a phase comparison image388. The magnitude comparison image 384 may include a ratio of eachvoxel density or intensity between the current heat image 160 and theprior heat image 150. The pixel or voxel intensities may be displayed inthe magnitude comparison image 384 for viewing by the user 25, such ason the display 22. It is understood, however, that the generatedcomparison image 380 may simply be used for analysis by the workstation42 to identify a bubble, if present, and compensate therefore.

The generated comparison images 380 may also include the phasecomparison image 388. As discussed above, the image data acquired withthe MRI system 26 may acquire different types of data including themagnitude image data, as illustrated in the magnitude comparison image384 and phase encoded image data as shown in the comparison image 388.

As illustrated in FIG. 7 , a hole or dark region 166 is present in theimage 160. The resultant comparison images may also include or identifya magnitude ratio where the magnitude comparison image 384 includes adark or low intensity region 392. The low intensity ratio region 392illustrates that there is a small ratio between the current heat image160 and the prior heat image 150. In various embodiments, as discussedfurther herein, a magnitude threshold may be used to assist indetermining whether a data set, such as the comparison data set 380,includes a bubble. A magnitude threshold may be about 0.20 to about0.90, and further include about 0.50 to about 0.750, and further includeabout 0.65. In various embodiments, therefore, a decrease in signal ofabout 32% to about 40%, and further including about 35%, may be used toassist in identifying a relevant comparison data set for including abubble.

Further, the phase comparison image 388 may also include a region ofphase differentiation or comparison 398. The phase comparison region 398may also illustrate the phase variations between the current heat image160 and the previous heat image 150. Thus, both a magnitude and a phasedifference may occur between the current heat image 160 and the priorheat image 150 when a bubble occurs in the subject 28.

While FIG. 7 illustrates an example of a magnitude comparison image 384and a phase comparison image 388, the bubble image library that isaccessed in block 194 may be compared to the comparison image data 380to assist in determining and/or to automatically identify a bubble, ifone is present in the comparison image data. Turning reference to FIG. 8the comparison image data 380 may include the magnitude comparison image384 and the phase comparison image 388. The comparison image data 380may be compared to the accessed bubble library in block 220, asillustrated in FIG. 4 . As schematically illustrated in FIG. 8 , abubble image library 420 is illustrated. The bubble image library 420may include a plurality or array of magnitude bubble model 424 and anarray or plurality of phase bubble models 428.

In the bubble image library 420, the array of magnitude bubble images424 may include selected number of bubbles, such as including a rangebetween a bubble having a two voxel radius in a first block 424 a and abubble having a twelve voxel radius in cell 424 b. It is understood thata no bubble cell (e.g. no phase difference) 424 c may also be present inthe library 420.

Similarly, the phase bubble image library 420 may also include phasebubble images for a plurality of diameters including a two voxeldiameter cell 428 a, a twelve voxel diameter cell 428 b, and a no bubblecell 428 c. As discussed above, the bubble image library 420 may alsoinclude a plurality of bubble images for bubbles rotated relative to theimage axis B₀. Accordingly, the bubble image library 420, as exemplaryillustrated in FIG. 8 , is merely exemplary of the plurality of bubbleimages that may be accessed in the bubble image library in block 194.

Regardless of the number of bubble images accessed in the library 194,which may be compared, either a selected number or sub-plurality of allof the bubble images from the bubble image library may be compared inblock 220. As illustrated in FIG. 8 , each of the bubble images from thelibrary 420 may be compared to the magnitude comparison image 384 asillustrated by comparison lines 450 a and 450 b.

As illustrated in FIG. 8 , the magnitude image comparison may be made tothe magnitude comparison image 384 and may allow for the generation of acorrelation image data set or array 460. The correlation array 460 mayinclude representation of a correlation between each of the images inthe bubble image library 420 to the comparison image data set 380.Accordingly, the correlation image array 460 may also includecorrelation regarding the phase bubble images, as illustrated by thecomparison lines 454 a and 454 b. The comparison lines illustrate thefirst and last bubble image being compared to the comparison image dataset 380. Accordingly, the correlation array 460 may include the samenumber of cells as the bubble image library, where each cell representsa comparison of the respective cell in the bubble image library. Thefirst cell 460 a includes a correlation of the first magnitude cell 424a to the comparison magnitude image 384 and the first phase library cell428 a to the comparison phase image 388. The correlation array 460 thatincludes cells relating to each of the library images, such as includinga largest radius correlation cell 460 b and a no bubble cell 460 c.Accordingly, the correlation array 460 may include correlation betweenall of the bubble images to the comparison image data 380.

The bubble image library 420 may include the bubble images of bubbles ofselected sizes and/or orientations. Further, the bubble images may becropped to a selected dimension, such as one or two pixels greater thanthe bubble model. Accordingly, the dimensionality of the bubble imagesmay be less than the size of the comparison image 380. To perform thecomparison, therefore, the bubble image from the bubble image library420 may be moved in a step wise manner across the comparison image 380.

A correlation between the bubble image from the bubble image library 420and a portion of the comparison images 380 will cause a highcorrelation, which may be depicted as a bright pixel or voxel in thecorrelation image in the correlation image array 460. That is, asillustrated in FIG. 8 , each of the bubble images may have a selectedgeometry or intensity or phase deviation, in the respective bubbleimages of the bubble image library 420. As the bubble image from thebubble image library 420 is compared to a portion of the comparisonimage 380, each of the pixels or voxels may include a selectedcorrelation. The correlation may be low or high. A high or largecorrelation may be indicated as a high intensity or high correlationwhich may be illustrated in the correlation array 460. Again, it isunderstood, that the correlation data and the correlation array 460 maybe illustrated for use by the user 25 and/or used in the system foridentification of the bubble. Nevertheless, high correlations may beidentified between the bubble images from the bubble image library 420and the comparison images 380.

In various embodiments, the bubble images in the bubble image libraryare masked to the voxels with greater than a 0.1 radian phase shift.This masking assists in localizing correlations between the bubble imagelibrary image and the comparison images. In addition, the crosscorrelations may be normalized by mean squared amplitude of the bubbleimages from the bubble image library to allow for correlations to becompared between library entries. In various embodiments, thecorrelation may be a comparison and may occur in the Fourier domain,particularly for complex value inputs of the comparison images.

In various embodiments generation of bubble images in the bubble imagelibrary may include non-square voxels, since imaging resolution may bedifferent in different dimensions. Also, bubble rotation may take placebefore or after synthesis of the bubble image, thus bubble coordinatesmay rotated before calculating the image, or rotate the image afterward.

The bubble image library may also be processed using a technique such assingular value decomposition or principle component analysis, to reduceits dimension for more efficient computation. In other words, instead ofdirectly calculating correlations between the comparison image and eachbubble image library entry, correlations may be determined between thecomparison image and a smaller number of optimized linear combinationsof bubble image library entries.

The correlation for each of the correlation images in the correlationarray 460 may be given a correlation score S_(ij) denoted by Equation 6,

$S_{ij} = {\max\left( {\frac{{❘X_{ij}❘} - {❘X_{ij}^{b}❘}}{1 - {❘X_{ij}^{b}❘}},0} \right)}$

In Equation 6, the correlation score may be a maximum of a correlationbetween the bubble image having a selected radius i and angle j, foreach of the bubble images from the bubble library. As noted in Equation6, the correlation score may attempt to remove background noise byproviding a correlation X_(ij) ^(b) that is a correlation between eachof the bubble images in the bubble image library and a tissue mask. Thetissue mask may be based upon an initial image, such as an image priorto any ablation or therapy being applied to the subject 28 and/or aninitial heat image. Accordingly, a mask may be used to remove falsecorrelations that may occur in the image. For example, in variousembodiments, heat formation in the subject 28 may cause phase change orphase deviation that may confound the bubble detection. Accordingly,masking the image or removing background may assist in achieving agreater bubble detection accuracy. It is understood that the optionaltissue mask may also be formed with the immediate previous accessed heatimage from block 202. Accordingly, a mask may include image data or acorrelation based upon possible heat that cause a phase change over thecourse of the treatment.

The bubble image library may have the bubble images formed at a selectedresolution that may be substantially greater than the resolution of thecomparison images. The resolution of the bubble images may be at aresolution great enough to allow for a detailed generation of the bubbleimages for comparison to the comparison images. Accordingly, during orafter the generation of the correlation image array 460, the correlationimage array, including the images therein, and/or the comparison images380, if upscaled, may be low pass filtered with a selected Gaussianfunction or kernel, such as a normalized Gaussian kernel. The resolutionof the comparison image 380 and the correlation image 460 may be reducedto a resolution similar to that of the acquired image data, such as thecurrent heat image from block 194.

After the low pass filter, pixels within the correlation images may beidentified as bubble pixels if the pixel or voxel has a magnitude thatis below a selected magnitude, if selected. As discussed above, a ratiomagnitude of 0.65 may be a selected threshold. Accordingly, if a voxeldoes not have a signal reduction of at least 35%, it may not be includedin a possible bubble detection. In addition, if the signal in a voxelincreases rather than decreases, it may not be included in a possiblebubble determination. Further, voxels having a selected correlationscore of at least 0.2, as discussed above, may also not be included in abubble detection. The correlation score may have any appropriate value,such as 0.3, 0.4, or higher. A selected higher maximum may reduce anumber of voxels selected to possibly be within a bubble. Accordingly,voxels that meet at least these two requirements may be included in abubble detection. As illustrated in FIG. 8 , the correlation images 460may be used to identify an image or one of the correlation images ashaving a voxel or group of voxels that are within a bubble asillustrated by 490 a and/or 490 b. The images that may be included in abubble may then be confirmed or processed, as discussed further herein.

The comparison and determination of the correlation images orcorrelations 460 may be executed instructions, such as with theprocessor system 40. Thus, the correlations 460 may be determinedsubstantially automatically based on the instructions formed based onthe disclosed method and process.

Further, as discussed above, the comparison of the bubble images fromthe bubble image library 420 may be made to the comparison images 380.However, as discussed above, the determination of a bubble may berelevant at or near the instrument 24 within the subject 28.Accordingly, the comparison image 380 may be reduced in dimensionality,such as by identifying a region of interest (ROI) within the comparisonimage 380 and/or the heat image. In various embodiments, the instrument24 may be navigated by being tracked with a selected tracking system, asdiscussed above.

As the current heat image accessed in block 198 may be generated withthe imaging system 26, the position of the instrument 24 within theimage data may be determined, as discussed above. Thus, the comparisonof the bubble images from the bubble image library may be minimized to aselected area or volume around a distal end of the instrument 24 withinthe subject (such as when the subject is registered to the image), suchas the comparison image 380. The amount of the image for comparison tothe bubble images may be selected to be only within a selected volume orarea relative to the tracked location of the instrument.

In addition or alternatively thereto, the user 25 may also identify aregion of interest for comparison to the bubble images from the bubbleimage library 420. The user 25 may identify the ROI by one or more inputdevices, such as the keyboard 44. In various embodiments, the user 25may draw or identify the ROI on the image 23 displayed with the displaydevice 22. Accordingly, an optional area or volume of a region ofinterest may be identified for a comparison in block 220. The comparisonof the bubble image to the generated comparison image may be in eitherone or both of the whole image and/or a selected region of interest. Theregion of interest, as noted above, may be based upon selection by theuser 25, a tracked location of the instrument 24 such as being trackedwith the navigation system, or an inherently registered position of theimage relative to the subject 28. For example, the ROI may be within avolume that is about 0.1 cm to about 5 cm from a selected location ofthe end of the instrument 24. Nevertheless, the comparison of the bubbleimage may be made to an appropriate portion of the comparison image fordetermining whether a bubble is present within the image.

Returning reference to FIG. 4 , after identifying a location of a bubblein the comparison image, a determination of whether to compensate or notmay be made in block 248. If no compensation is determined, as discussedabove various steps may be followed, such as pausing therapy to allowthe bubble to dissipate. However, if compensation is determined, theYES-path 260 may be followed to the removed distortion/artifact causedby a bubble from the current heat image in block 270.

The compensation may include the removal of the distortion, such asphase variance, caused by a bubble in the heat image and/or thecomparison image. In various embodiments, therefore, the compensationmay include a subtraction of the bubble image from the bubble imagelibrary that most matches the identified bubble. Thus, removing thebubble distortion as the bubble image from the bubble image library thatis identified in the generated comparison images may be removed. Thebubble image may be removed as being placed on the heat image or thecomparison image as a determined center of the identified bubble in theimage. In various embodiments, the center may be a weighted mean centerin the image. The bubble may be subtracted or removed from the image byremoving the information of the bubble image from the bubble imagelibrary from the heat image.

In various embodiments, with continuing reference to FIG. 4 andadditional reference to FIG. 9 , the removed distortion in block 270 isillustrated an alternative and/or greater detail. As noted above, theremoved distortion may be identified or determined to be a sub-routineas a part of the method 180. As also noted above, the remove distortion270 and temperature determination 274 may be executed instructions, suchas with the processor system 40. Thus, the distortion removal andcompensation may be determined substantially automatically based on theinstructions formed based on the disclosed method and process.

Accordingly, with reference to FIG. 9 , the removed distortion method orsub-routine is described in greater detail. Once the bubble isidentified in block 244, all of the voxels in the comparison image thatare part of the bubble and/or likely part of a bubble may be identified.Accordingly, all of the voxels inside of the bubble (i.e. as identifiedby the bubble image from the bubble image library accessed in block 194)may have a dipole field calculated for each voxel centered at each ofthe voxels. The dipole field may be generated as a matrix, which may bereferred to as matrix A, and be defined by Equation 7,

$\frac{\left( {\left( {x - {xc}} \right)^{2} - \left( {y - {yc}} \right)^{2}} \right)}{\left( {\left( {x - {xc}} \right)^{2} + \left( {y - {yc}} \right)^{2}} \right)}$

Equation 7 is the difference of the squared x and y coordinates in theimage divided by their sums. The coordinates are centered at the voxellocation identified as xc and yc. Accordingly, the calculation of thedipole field may be made in block 480. The dipole field is a map basedupon the x and y locations within the image and may be formed intovectors in block 484. The vectors may be formed into two columns of amatrix. The dipole matrix may then be used to analyze the comparisonphase image 388, as discussed above in FIG. 7 and FIG. 8 .

The dipole matrix may be fitted to the phase comparison phase image,such as the image 388, in block 490. The fitted phase image may besubtracted from the current heat image in block 494. The subtraction ofthe comparison phase image 388 that is fitted with the dipole matrix maybe used to determine the proper heat or the phase change due to heatwithin the current heat image that is not affected by the bubble.

The dipole matrix may be used to identify or clarify the voxels in thecurrent heat image that are phase distorted caused by the bubble ratherthan a phase change due to heating of the tissue within the subject 28.Accordingly, subtracting the comparison phase image fitted with thedipole matrix from the current heat image removes the phase distortioncaused by the bubble, rather than heat. Thus, the removeddistortion/artifact of the bubble in block 270 may allow for adetermination of the temperature in block 274 at all of the voxelswithin the current heat image 198.

With continuing reference to FIG. 9 , and with returning reference toFIG. 4 , the determination of the temperature in the current heat imagemay be based upon the removal of the phase distortion caused by thebubble. Accordingly, once the bubble phase is removed, the temperaturemay be determined in block 274. Further, with reference to FIG. 9 , thetemperature determination may include various sub-steps or asub-routine. For example, temperature determination in block 274 mayinclude a temperature unwrapping in block 510. The temperatureunwrapping in block 510 may include correcting for phase wrap when phaseencoding the heat determination image accessed in block 198.Accordingly, temperature unwrapping may incur, due to the phase, inblock 510.

The temperature determination may also include drift removal in block520. Drift removal may include determination of a temperature drift overtime. Temperature drift over time may occur for various reasons, anddrift removal may include determination of a temperature drift overtime, such that the accumulation of phase drift is monitored andtemperature data are adjusted for this drift artifact across the imageanatomy. Accordingly, a summation of all heat images may be made todetermine a masking and/or subtraction of heat drift that may haveoccurred prior to the current heat image accessed in block 194. Otherappropriate methods to determine drift and/or for its removal may alsobe used. For example, a drift correction may be derived from theinstantaneous heating image (e.g. the current heat image), by fitting alow order polynomial to the entire phase difference image (e.g. thephase portion of the comparison image (i.e. phase variance image 388,and then subtracting it out of the temperature map that is based on thecurrent heat image.

Finally, a temperature map may be made in block 530 based upon theremoval of the bubble phase distortion and accounting for optionaladditional features such as temperature unwrapping and drift removal inblocks 510, 520, respectively, as discussed above. The temperature mapmay include a determined temperature for each voxel in the current heatimage access in block 198. The temperature determination may alsoinclude or be a temperature differential from a previous heat image.Further, as noted above, the determination may be based upon informationcollected with the image data acquired with the imaging system 26 of thesubject 28. In various embodiments, the information may include phasechange or other information, such as relaxation times, for each voxel inthe image. In various embodiments, the determination of the temperaturemay be performed according to generally known techniques, such as thoseused in the Visualase® cooling laser fiber system sold by Medtronic,Inc. The temperature map created in block 530, however, may be madeafter a removal of a bubble or possible bubble that is identified in thecurrent heat image, according to the method 180, including the varioussub-steps as noted above.

Accordingly, the procedure may be performed on a subject and atemperature may be determined with an image, as discussed above. Thetemperature may be determined regardless of whether the formation of abubble occurs or not, including or based upon the method as noted above.Thus, a bubble may occur in an image, it may be automatically identifiedaccording to instructions executed with a processor based upon thealgorithm noted above, and a corrected or undistorted temperature mapmay be generated based upon the current heat image. Thus the user 25 maydetermine or have determined the temperature map for the subject.

Returning reference to FIGS. 1 and 2 and with further reference to FIG.4 , at least one bubble image from the accessed bubble image library maybe compared to the comparison image in block 220. In comparing the atleast one bubble image library, as noted above, all of the images in theimage bubble library may be compared to the comparison bubble image. Asnoted above, each of the bubble images may include selected pixels orvoxels (based upon the type of image generated and the comparisonimage), for allowing for a comparison between the bubble image and thecomparison image. Generally, a pair wise comparison between pixelsand/or voxels in the bubble image is made with pixels and/or voxels inthe comparison image. To compare the bubble image to the entire heatimage, however, may include extraneous or superfluous correlationsand/or may increase analysis time. In various embodiments, therefore, asnoted above, a region of interest (ROI) may be determined for limitingor defining only an area or volume in which the comparison of the bubbleimage is made to the comparison image. In various embodiments, the ROImay be determined based upon navigating the instruments 24 in thesubject 28.

With additional reference to FIG. 10 , a navigated determined region ofinterest 600 is illustrated. The navigated determined region of interestmay be incorporated into the method 180 illustrated in FIG. 4 , such asimmediately prior to the compared at least one bubble image from theaccessed bubble image library to the current image in block 220. It isfurther understood that, as illustrated in FIG. 4 , the determination ofthe ROI may be a sub-routine incorporated into the comparison in block220. Accordingly, the determined ROI 600 may be understood to be asub-routine incorporated into the method 180. Thus, as noted above, themethod 600 may be executed instructions, such as with the processorsystem 40. Thus, the method 600 may be determined substantiallyautomatically based on the instructions formed based on the disclosedmethod and process.

Generally, when navigating the instrument 24 during a selectedprocedure, the instrument 24 may be tracked with a selected trackingsystem, such as the tracking system 50 discussed above, to determine alocation of at least a portion of the instrument 24. Accordingly, thedetermined ROI method 600 may begin within the method 180 with thecomparison block 210 and proceed to track the instrument in block 614.

In tracking the instrument in block 614, a location of the instrument 24may be determined by the navigation system 20. The location of theinstrument 24 may be determined relative to the subject 28, such as withthe DRF 58. As noted above, the images of the subject 28, including theimage 23, may be registered to the patient 28. In various embodiments,the image 23 may be registered to the subject 28 in block 618.Accordingly, the tracked location of the instrument 24 may be knownrelative to the image 23 based upon tracking the instrument in block614. Registration may occur in any appropriate manner, including thosediscussed above, such as with identifying fiducial points in the subject28 and the image 23 (fiducials may be natural or implantedartificially). Regardless, the image may be registered in block 618.

Thus, the tracked position of the instrument in block 614 may bedetermined relative to the image in block 622. In determining thelocation of the instrument in block 622, a region within the image 23may be identified in the image space. As noted above, at least a portionof the instrument may have its location determined, such as the terminalend 110 of the instrument 24 and/or the distal end 104 of the energydelivery device 100. The location of the portion of the instrument, suchas the terminal end of the fiber optic member or energy deliveringdevice 100, may be used to identify a region relevant for temperaturedetermination.

A determined region of interest may be based upon a determined locationof the instrument in block 622 by determining a region of interest inblock 628. The determined region of interest may include a selected areaor volume around or near the determined location of the instrument orportion of the instrument. For example, a determined region of interestmay be defined as a volume having a radius of a selected length (e.g.about 1 cm to about 6 cm and/or about 2 pixels or voxels to about 12pixels or voxels). The region of interest may be centered on or near thedetermined location of the portion of the instrument and may bedetermined in block 628.

In various embodiments, the processor, such as the processor system 40discussed above, may recall a predetermined or determine a size of aregion of interest. It is understood, however, that the user 25 may alsodefine a region of interest relative to the tracked position of theinstrument and the determined position of the instrument in block 622.Accordingly, determining the region of interest in block 628 may beidentifying the portion of the image 23 (e.g. tracked center of theheating portion of the instrument and a volume in a selected radiustherefrom).

As noted above, the determining of the ROI may be a sub-routine of theblock 220. As illustrated in FIG. 10 , however, the determination of theROI 600 may be inserted between the generated comparison image in block210 and the compared at least one bubble image from the accessed bubbleimage library to the current comparison image in block 220. Accordingly,the determined ROI with the navigation of the instrument 24 may beunderstood to be inclusive or included, as a selected option, within themethod 180.

Returning reference to FIG. 4 , as noted above, the comparison todetermine portion of an image as to whether a bubble is present orpossibly present in an image occurs in the method 180 at block 220. Asdiscussed above, with reference to FIG. 4 and FIG. 5 , a bubble imagemay be generated/or accessed for comparison to the comparison image. Thebubble image may be based upon a model of a bubble and an image of themodel bubble, including a magnitude and phase variance. In variousembodiments, however, in addition to the bubble image model and/oralternatively thereto, a bubble may be identified and/or a possiblebubble may be identified by analysis of the comparison image directly.In various embodiments, a heuristic method may be applied in addition toand/or alternatively to the bubble image model as discussed above.

With continuing reference to FIG. 4 and additional reference to FIG. 11, a method 220 b is illustrated. The method 220 b may be an addition toand/or alternative to the comparison of the bubble image from the bubbleimage library, as discussed above. The comparison method 220 b, however,may be included in the method 180, as illustrated in FIG. 4 , todetermine whether a bubble is present in block 230 and the identifiedlocation of the bubble in the comparison image in block 244. Thus, thecomparison method 220 b may be included or understood to be asub-routine within the method 180, as discussed above.

Thus, the bubble image comparison algorithm or system, as discussedabove and illustrated in various figures such as FIG. 6 and FIG. 8 , mayalso be an alternative and/or addition to the method 220 b. The methodillustrated in FIG. 6 and FIG. 8 including the bubble image library mayalso be understood to be a sub-routine of the method 180.

The heuristic or non-model comparison 220 b may begin at block 210, asdiscussed above. The comparison image may be generated in block 210 andreceived for comparison in block 660. The received comparison image orimages may include the comparison image data 668, as illustrated in FIG.12 . The comparison image data 668 may be similar to the image data 380,as discussed above. Generally, the comparison image data may be a ratioof the current heat image 160 and the prior or previous heat image 150.As discussed above, the current heat image 160 may or may not includeone or more voxels or pixels that include a selected or have a selectedchange when compared to the prior heat image 150. In variousembodiments, as discussed above, the comparison image data 668 may bebased upon a ratio of the current heat image 160 and the previous heatimage 150. As also discussed above, the comparison image data 668 mayinclude a magnitude image data 670 and a phase variance image data 674.As exemplary illustrated in FIG. 12 the magnitude image data 670 mayinclude a region of magnitude change or decrease 678 and the phasevariance initiated 674 may include a phase variance region or area 682.

In the comparison method 220 b, a filter 692 may be moved over thecomparison image in block 688. The filter may be defined and/or saved ina selected memory, such as in the memory 46. The processor system 40 maythen recall the filter and compare it or move it over the comparisonimage data 668, as discussed further herein.

The filter may be defined to attempt to identify or to identify clustersor localized regions of voxels or pixels that include selected criteriaor variances. The variances may be predefined and included within thefilter stored in the memory 46. In various embodiments, however, theuser 25 may also identify selected features or criteria to include inthe filter for comparison to the comparison image in block 688.

The filter may include a selected size such as about 2 voxels to about15 voxels, including about 7 voxels to about 11 voxels, and furtherincluding about 9 voxels. The filter may have a selected dimension,therefore, and may be moved within a selected dimension of thecomparison image. As discussed above the filter may be moved within theentire image. In various embodiments, however, the filter may also bemoved within a region of interest. As noted above, the region ofinterest may include a manually selected region of interest (e.g. aregion of interest identified by the user 25, such as by drawing oridentifying with an input a ROI in the image 23) and/or automaticallydetermined based upon selected features, such as within the ROIdetermination 600 illustrated in FIG. 10 . Accordingly, it is understoodthat the filter may be applied to the comparison image in anyappropriate region, including the entire image or only a region ofinterest which may be less than the entire image.

The filter may be to determine or identify selected voxels within thecomparison image 668 that may include or be determined to be within abubble. The filter, therefore, may be applied to the comparison imagedata 668 by the processor system 40, in a manner similar to applying thebubble image as discussed above. Thus, the filter may be applied in asubstantially pairwise manner relative to the comparison image 668 todetermine a comparison and/or determination of whether a voxel meets aselected threshold, as discussed further herein.

The filter 692, as illustrated in FIG. 12 may be illustrated as an areaor volume filter 692, as discussed above. In various embodiments, thefilter may include or be inclusive of at least two features or criteria,but are illustrated separately in FIG. 12 . For example, in themagnitude image 670, the filter 692 may include the selected dimensions,as discussed above, and identify or be compared to the magnitude image670 to determine a selected signal drop. The selected signal drop mayinclude or be defined as a magnitude change in the ratio or comparisonimage data 668 of a voxel of at least about 0.5 to about 0.95, andfurther including about 0.7 to about 0.9, and further including amagnitude variance in the comparison image 668 of about 0.8. In otherwords, the filter may identify a signal decrease of about 20% from theheat image to the current heat image as slightly being inclusive withina bubble.

The filter 692 b may include a second criterion that is compared ormoved across the phase variance image 674. The filter may identify inthe phase variance image 674 voxels that have a phase variance of about0.5 radians to about 1.5 radians, further including about 1 radian. Thephase variance may be identified or determined on a per-voxel basis,such as in a pair wise comparison between the filter 692 b and thevoxels in the phase variance image 674.

The filter 692, therefore, is moved over or compared to the comparisonimage data 668, including either the entire image and/or within a regionof interest, as discussed above. Based upon the evaluation of the voxelswithin the filter, a determination of whether identified voxels arepossibly within a bubble is made in block 698. As discussed above, thefilter may be used to identify voxels that are possibly within a bubblebased upon the selected criteria and/or thresholds noted above,regarding the magnitude and phase variance. All of the voxels identifiedas possibly within a bubble, based upon the filter 692, may then bedetermined or saved in block 698. Generally, a voxel may be determinedto possibly be within a bubble if the voxel meets both criteria, such ashaving a magnitude variance of about 0.8 (i.e. signal decrease of about20%) and a phase variance of about 1 radian.

Once the voxels are determined or identified to be possibly within abubble in block 698, a dimension of voxels within a selected distance ofone another may be made in block 702. As discussed above, the filter 692may be used to determine whether selected voxels or whether voxels havea selected magnitude change (e.g. signal drop) and/or phase variance.Generally, a voxel determined to be possibly within a bubble will berequired to include both thresholds, as discussed above.

A bubble may be determined to have a selected dimension and/or geometry.For example, a bubble may be assumed to have a radius of at least about2 voxels and/or equal to or less than about 12 voxels. Accordingly,determining a dimension of a cluster of voxels in block 702 may be usedto identify whether a bubble is present in the comparison image 668. Acluster may be voxels that meet the bubble filter criteria that areadjacent (e.g. touching) one another or within a selected distance (e.g.0.5 voxels apart). All voxels that meet the distance criteria may beidentified as a cluster. Once the dimension of any cluster of voxels isdetermined in block 702, a determination of whether a bubble is presentin block 230 may be made.

The determination of whether a bubble is present in the comparison imagein block 230, based upon the heuristic comparison 220 b, may includewhether any determined voxels in block 698 meet an identified orselected dimension in block 230 once the cluster has been determined inblock 702. Accordingly, if the cluster of voxels has been identified andincludes a dimension of at least 2 voxels, a determination that a bubbleis present in the comparison image may be made in block 230. Accordinglythe YES-path 238 may be followed, as illustrated and discussed in FIG. 4above.

If no cluster or no cluster of voxels is not determined to meet a sizecriterion, such as less than 2 voxels and/or greater than 12 voxels,determination that a bubble is not present in the image may be made indetermination block 230 and the NO-path 234 may be followed. It isunderstood that the dimension of a cluster may be predetermined, andincluded in the filter, for analysis by the processor system 40. It isalso understood that the user 25 may also input a selected clusterdimension for analysis of the comparison image 668. Accordingly, thecomparison method 220 b may be used to compare and/or assist inidentifying or determining whether a bubble is present in the comparisonimage. Either alone and/or in combination to the comparison with thebubble image library image, as discussed above.

Example embodiments are provided so that this disclosure will bethorough, and will fully convey the scope to those who are skilled inthe art. Numerous specific details are set forth such as examples ofspecific components, devices, and methods, to provide a thoroughunderstanding of embodiments of the present disclosure. It will beapparent to those skilled in the art that specific details need not beemployed, that example embodiments may be embodied in many differentforms and that neither should be construed to limit the scope of thedisclosure. In some example embodiments, well-known processes,well-known device structures, and well-known technologies are notdescribed in detail.

Instructions may be executed by a processor and may include may includesoftware, firmware, and/or microcode, and may refer to programs,routines, functions, classes, data structures, and/or objects. The termshared processor circuit encompasses a single processor circuit thatexecutes some or all code from multiple modules. The term groupprocessor circuit encompasses a processor circuit that, in combinationwith additional processor circuits, executes some or all code from oneor more modules. References to multiple processor circuits encompassmultiple processor circuits on discrete dies, multiple processorcircuits on a single die, multiple cores of a single processor circuit,multiple threads of a single processor circuit, or a combination of theabove. The term shared memory circuit encompasses a single memorycircuit that stores some or all code from multiple modules. The termgroup memory circuit encompasses a memory circuit that, in combinationwith additional memories, stores some or all code from one or moremodules.

The apparatuses and methods described in this application may bepartially or fully implemented by a special purpose computer created byconfiguring a general purpose computer to execute one or more particularfunctions embodied in computer programs. The computer programs includeprocessor-executable instructions that are stored on at least onenon-transitory, tangible computer-readable medium. The computer programsmay also include or rely on stored data. The computer programs mayinclude a basic input/output system (BIOS) that interacts with hardwareof the special purpose computer, device drivers that interact withparticular devices of the special purpose computer, one or moreoperating systems, user applications, background services andapplications, etc.

The computer programs may include: (i) assembly code; (ii) object codegenerated from source code by a compiler; (iii) source code forexecution by an interpreter; (iv) source code for compilation andexecution by a just-in-time compiler, (v) descriptive text for parsing,such as HTML (hypertext markup language) or XML (extensible markuplanguage), etc. As examples only, source code may be written in C, C++,C#, Objective-C, Haskell, Go, SQL, Lisp, Java®, ASP, Perl, Javascript®,HTML5, Ada, ASP (active server pages), Perl, Scala, Erlang, Ruby,Flash®, Visual Basic®, Lua, or Python®.

Communications may include wireless communications described in thepresent disclosure can be conducted in full or partial compliance withIEEE standard 802.11-2012, IEEE standard 802.16-2009, and/or IEEEstandard 802.20-2008. In various implementations, IEEE 802.11-2012 maybe supplemented by draft IEEE standard 802.11ac, draft IEEE standard802.11ad, and/or draft IEEE standard 802.11ah.

A processor or module or ‘controller’ may be replaced with the term‘circuit.’ The term ‘module’ may refer to, be part of, or include: anApplication Specific Integrated Circuit (ASIC); a digital, analog, ormixed analog/digital discrete circuit; a digital, analog, or mixedanalog/digital integrated circuit; a combinational logic circuit; afield programmable gate array (FPGA); a processor circuit (shared,dedicated, or group) that executes code; a memory circuit (shared,dedicated, or group) that stores code executed by the processor circuit;other suitable hardware components that provide the describedfunctionality; or a combination of some or all of the above, such as ina system-on-chip.

The foregoing description of the embodiments has been provided forpurposes of illustration and description. It is not intended to beexhaustive or to limit the disclosure. Individual elements or featuresof a particular embodiment are generally not limited to that particularembodiment, but, where applicable, are interchangeable and can be usedin a selected embodiment, even if not specifically shown or described.The same may also be varied in many ways. Such variations are not to beregarded as a departure from the disclosure, and all such modificationsare intended to be included within the scope of the disclosure.

Further areas of applicability of the present teachings will becomeapparent from the detailed description provided above. It should beunderstood that the detailed description and specific examples, whileindicating various embodiments, are intended for purposes ofillustration only and are not intended to limit the scope of theteachings.

What is claimed is:
 1. A method of selecting a region for determining apresence of a bubble in an image, comprising: determining a trackedlocation of an instrument positioned within a subject in a subject spacedefined by the subject; accessing a current image of the subject inwhich the instrument is positioned; registering an image space of thecurrent image to the subject space of the subject; determining alocation of the instrument within the image space based on thedetermined tracked location of the instrument; determining a region ofinterest relative to the determined location of the instrument withinthe image space; and analyzing the current image to determine if abubble is present in the current image within the region of interest. 2.The method of claim 1, further comprising: tracking the instrumentwithin the subject space with a tracking system.
 3. The method of claim2, further comprising: operating the tracking system to determine thetracked location separate from an imaging system configured to acquirean image of the subject.
 4. The method of claim 3, further comprising:acquiring the current image with the imaging system.
 5. The method ofclaim 3, further comprising: generating the current image as acomparison image generated by comparing a first image and a secondimage; wherein the first image and the second image are acquired withthe imaging system.
 6. The method of claim 1, wherein analyzing thecurrent image to determine if the bubble is present in the current imagewithin the region of interest, comprises: comparing at least one bubbleimage to the current image; determining a correlation between the atleast one bubble image and the current image; and outputting acorrelation value based on the determined correlation.
 7. The method ofclaim 6, further comprising: accessing a bubble image library includinga plurality of bubble images.
 8. The method of claim 6, whereinanalyzing the current image to determine if the bubble is present in thecurrent image within the region of interest, further comprises:selecting a first region within the region of interest; performing apairwise comparison of voxels from the at least one bubble image to theselected first region; selecting a second region within the region ofinterest; and performing a pairwise comparison of voxels from the atleast one bubble image to the selected second region.
 9. The method ofclaim 1, wherein determining the tracked location of the instrumentpositioned within the subject in the subject space defined by thesubject, comprises: associating a tracking device with the instrument;and operating a tracking system to track the tracking device associatedwith the instrument.
 10. The method of claim 1, wherein determining theregion of interest relative to the determined location of the instrumentwithin the image space, comprises: determining a dimension of the regionof interest; and centering the region of interest on the determinedtracked location of the instrument.
 11. The method of claim 10, whereinthe determined tracked location of the instrument includes the trackedlocation of only a portion of the instrument.
 12. A system to select aregion for determining a presence of a bubble in an image, comprising: atracking system operable to track a tracking device; a navigation systemoperable to: determine a tracked location of an instrument positionedwithin a subject in a subject space defined by the subject, wherein thetracking device is associated with the instrument, access a currentimage of the subject in which the instrument is positioned, anddetermine a location of the instrument within the image space based onthe determined tracked location of the instrument and a registration ofthe subject space and an image space of the current image; and an imageanalysis processor system operable to execute instructions to: identifya region of interest relative to the determined location of theinstrument within the image space, and analyze the current image todetermine if a bubble is present in the current image within the regionof interest.
 13. The system of claim 12, further comprising: a displaydevice to display a heat map image of the subject based on the currentimage.
 14. The system of claim 13, further comprising: an imaging systemoperable to acquire a first image and a second image; wherein the imageanalysis processor system is operable to execute further instructions togenerate the current image by comparing the first image and the secondimage.
 15. The system of claim 13, further comprising: a memory systemhaving stored thereon a bubble image library including a plurality ofbubble images; wherein the image analysis processor system is operableto recall the bubble image library.
 16. The system of claim 15, whereinthe image analysis processor system is operable to execute furtherinstructions to analyze the current image to determine if the bubble ispresent in the current image within the region of interest by: comparingat least one bubble image recalled from the bubble image library to thecurrent image; determining a correlation between the at least one bubbleimage and the current image; and outputting a correlation value based onthe determined correlation.
 17. The system of claim 12, wherein theimage analysis processor system is operable to execute furtherinstructions to identify the region of interest by determining a regioncentered on the determined location of the instrument within the imagespace.
 18. A method of selecting a region for determining a presence ofa bubble in an image, comprising: accessing a first image and a secondimage of the subject in which an instrument positioned; generating acomparison image by comparing the first image and the second image;determining a location of the instrument within an image space of atleast the comparison image based on a determined tracked location of theinstrument; determining a region of interest relative to the determinedlocation of the instrument within the image space; and correcting atleast the second image for determining a temperature of at least theregion of interest in the second image.
 19. The method of claim 18,further comprising: tracking the instrument in the subject space; andregistering an image space of the comparison image to the subject spaceof the subject.
 20. The method of claim 19, further comprising:comparing a plurality of bubble images to the comparison image;determining a correlation between at least one bubble image of theplurality of bubble images and the comparison image; and outputting acorrelation value based on the determined correlation of the at leastone bubble image.